Rapise 1.5
You can use the special SeSFindObj('object name') command to perform the check:
if(SeSFindObj('Object Name'))
{
// Object is available
}
else
{
// Object is not available
}
Rapise 7.0+
If you have a web test then you may use Navigator.CheckObjectExists that also accepts XPath as a parameter.
Also in desktop, mobile and web tests one may use Global.DoWaitFor with timeout parameter set to 1.